Beam selection in millimeter wave systems

ABSTRACT

A beam selection method and apparatus suitable for millimeter wave (mmW) communication systems is disclosed. In one aspect, a user equipment (UE) may perform a beam sweep procedure to identify suitable downlink beams candidates from one or more gNBs. The UE may generate a beam list by selecting some of the downlink beams for active tracking. When beams on the beam list become unavailable, the UE may compare the number of available beams on the beam list with a threshold value. If the number of available beams falls below the threshold, the UE may perform another beam sweep procedure.

BACKGROUND

This application claims the benefit of U.S. patent application Ser. No.62/556,970 entitled “BEAM SELECTION IN MILLIMETER WAVE SYSTEMS” filed onSep. 11, 2017, which is expressly incorporated by reference herein inits entirety.

The following relates generally to wireless communication, and morespecifically to User Equipment (UE) beam search and selection inmillimeter wave systems.

Wireless communications systems are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be capable ofsupporting communication with multiple users by sharing the availablesystem resources (e.g., time, frequency, and power). Examples of suchmultiple-access systems include code division multiple access (CDMA)systems, time division multiple access (TDMA) systems, frequencydivision multiple access (FDMA) systems, and orthogonal frequencydivision multiple access (OFDMA) systems, (e.g., a Long Term Evolution(LTE) system, or a New Radio (NR) system). A wireless multiple-accesscommunications system may include a number of base stations (e.g., agNB, TRP, eNB) or other network access network nodes, eachsimultaneously supporting communication for multiple communicationdevices, which may be otherwise known as user equipment (UE).

In some wireless systems, base stations and UEs may communicate usingdirectional transmissions (e.g., beams), where beamforming techniquesmay be applied using one or more antenna arrays to generate beams indifferent directions. For example, a base station may transmit downlinkcommunications (e.g., synchronization signals, signals, data signals,etc.) to a UE using a transmit beam in a particular direction, and theUE may in turn receive the downlink communications on a receive beam inan opposite direction. In very high frequency systems a base station maytransmit using narrow beams to overcome path loss. A UE may be able toreceive on many suitable down-like beams from one or more gNBs.Searching and tracking a large amount of beams increases complexity andconsumes modem and RF power. It may thus be desirable to improvetechniques for downlink beam selection in beamformed communicationsystems.

SUMMARY

Beam training and beam selection in mmW systems is important. Beamtraining procedures take time, power, and increase modem complexity.Moreover beam training may reveal that there are many suitable beamsthat are available to the UE. Tracking, refining and reporting on manybeams also increases time, power and modem complexity.

Disclosed are various methods and apparatuses for executing beam searchprocedures and generating UE beam lists for tracking. One aspectfeatures a beam list generated from a beam training procedure. The beamlist contains beams (or beam pairs) that the UE may use for tracking.The UE may identify suitable beams for use and if the number of suitablebeams falls below a threshold number the UE may perform another beamtraining procedure. Other aspects include a beam list featuring beamsfrom multiple gNB. In various aspects, the beams chosen for inclusion onthe beam list may be chosen using a variety of different criteria. Thebeam list provides the UE with beams (or beam pairs) for tracking.Tracking multiple beams is important in case of beam blockage or linksignal degradation or outage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a system for wireless communicationthat supports mmW beam selection in accordance with aspects of thepresent disclosure.

FIG. 2 illustrates an example of a wireless communications system thatsupports mmW beam selection in accordance with aspects of the presentdisclosure.

FIG. 3 illustrates an example of a wireless communications system thatsupports mmW beam selection in accordance with aspects of the presentdisclosure.

FIG. 4 illustrates an example of the number of beams available to a UEin a serving cell by received SNR threshold.

FIG. 5 illustrates an example of the number of useful beams on a beamlist that might be available to a UE over time.

FIG. 6 a block diagram of a UE according to an aspect of the presentdisclosure.

FIG. 7 illustrates a block diagram of gNB according to an aspect of thepresent disclosure.

FIG. 8 illustrates an exemplary flow diagram of mmW beam trackingfunction in a UE.

FIG. 9 illustrates an exemplary flow diagram of gNB function forgenerating a UE beam tracking threshold.

FIG. 10 illustrates an exemplary flow diagram of mmW beam trackingfunction in a UE.

FIG. 11 illustrates an exemplary flow diagram of gNB function forchanging a code book.

DETAILED DESCRIPTION

Some wireless communication systems may support beamformed transmissionsbetween a base station and a user equipment (UE). For example, somesystems may operate in millimeter wave (mmW) frequency ranges, e.g., 28GHz, 40 GHz, 60 GHz, etc. Wireless communication at these frequenciesmay be associated with increased signal attenuation (e.g., path loss),which may be influenced by various factors, such as temperature,barometric pressure, diffraction, etc. As a result, signal processingtechniques, such as beamforming, may be used to coherently combineenergy and overcome path losses at these frequencies. A wireless devicemay use a number of antenna ports (e.g., 1, 2, 4, 8 antenna ports)associated with arrays of antennas to form beams in various directionsusing a number of analog weight factors. For example, as a base stationtransmits downlink signals using directional beams, a UE may alsoutilize beamforming for the UE's own directional receive beams (and itsuplink transmit beams for uplink transmissions to the base station).

A gNB may transmit SS blocks, CSI-RS signals or other downlink beamsignals on different directional downlink transmit beams. An SS blockmay be a combination of Primary Synchronization Signals (PSS), SecondarySynchronization Signals (SSS) and/or Primary Broadcast Channel Signals(PBCH). The PBCH may have Demodulation Reference Signals (DMRS) embeddedin them. The transmit beams may, over time, cover the geographiccoverage area of a cell allowing a UE inside the cell to synchronizewith the downlink transmit beams.

A UE in a serving cell may perform a beam training operation todetermine synchronization signals associated with different downlinkbeams that can be received and decoded. The UE may consider these beamsas candidates for a beam list that will be used for beam trackingpurposes. The list may also contain a receive beam for receiving thedownlink beams forming a beam pair. It can be appreciated that a UE maywant a beam list with multiple beam pairs to track in case of a blockingevent that would render one or more beam pairs unusable. It can also beappreciated that tracking multiple beams increases complexity, powerusage, and modem complexity. Accordingly, it may be important to limitor proactively manage the number of beams on the beam list. The beamlist may vary and result in different operating characteristics.

Moreover, beam training also increases complexity, power usage and modecomplexity. Accordingly it may make sense to limit beam training events.In one aspect, a UE in a serving cell may limit beam training bylimiting beam searches and performing a beam search only when the numberof useful beams in the beam list falls below a threshold. The thresholdmay be determined by the UE and/or the gNB based on a variety offactors.

In some aspects, the UE may put beams from multiple gNB on its beam listproviding rate and spatial diversity. In some aspects, serving cellbeams on the beam list may be limited to a small number (e.g. 1 to 3beams). These beams might be chosen for example to correspond to UEbeams in different subarrays indicating correspondence to other clustersin the channel. In some aspects the beam list may be generated based onUE distance to the gNB transmitting the beam. The UE might for examplefavor beams that are a short distance to the gNB with the number ofbeams from each gNB being an inverse function of the distance. Varioustechniques such as triangulation may be used to estimate distances.

Beam lists may also be populated or refined using other criteria. Forexample, a UE may favor beams that allow for detection and reasonabledemodulation performance with pseudo-omnidirectional (PO) beams allowingthe UE to save power. In some aspects, UEs may also request that a gNBuse a coarser codebook reducing the UE power requirements and AdjacentChannel Leakage Ratio (ACLR) levels.

Aspects of the disclosure are initially described in the context of awireless communications system. Examples are also provided whichdescribe various transmit and receive beam configurations for whichefficient transmit power control may be applied using one or more RACHbeam transmission counters. Aspects of the disclosure are furtherillustrated by and described with reference to apparatus diagrams,system diagrams, and flowcharts that relate to uplink transmit powercontrol during random access procedures.

FIG. 1 illustrates an example of a wireless communications system 100 inaccordance with various aspects of the present disclosure. The wirelesscommunications system 100 includes base stations 105, UEs 115, and acore network 130. In some examples, the wireless communications system100 may be a Long Term Evolution (LTE), LTE-Advanced (LTE-A) network, ora New Radio (NR) network. In some cases, wireless communications system100 may support enhanced broadband communications, ultra-reliable (i.e.,mission critical) communications, low latency communications, andcommunications with low-cost and low-complexity devices. Wirelesscommunications system 100 may support the use beam training proceduresallowing UEs 115 to determine gNB 105 beams that may be paired with oneor more UE beams. UEs 115 may select some of these beam pairs forinclusion on a beam list.

Base stations 105 may wirelessly communicate with UEs 115 via one ormore base station antennas. Each base station 105 may providecommunication coverage for a respective geographic coverage area 110.Communication links 125 shown in wireless communications system 100 mayinclude uplink transmissions from a UE 115 to a base station 105, ordownlink transmissions, from a base station 105 to a UE 115. Controlinformation and data may be multiplexed on an uplink channel or downlinkaccording to various techniques. Control information and data may bemultiplexed on a downlink channel, for example, using time divisionmultiplexing (TDM) techniques, frequency division multiplexing (FDM)techniques, or hybrid TDM-FDM techniques. In some examples, the controlinformation transmitted during a transmission time interval (TTI) of adownlink channel may be distributed between different control regions ina cascaded manner (e.g., between a common control region and one or moreUE-specific control regions).

UEs 115 may be dispersed throughout the wireless communications system100, and each UE 115 may be stationary or mobile. A UE 115 may also bereferred to as a mobile station, a subscriber station, a mobile unit, asubscriber unit, a wireless unit, a remote unit, a mobile device, awireless device, a wireless communications device, a remote device, amobile subscriber station, an access terminal, a mobile terminal, awireless terminal, a remote terminal, a handset, a user agent, a mobileclient, a client, or some other suitable terminology. A UE 115 may alsobe a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a tabletcomputer, a laptop computer, a cordless phone, a personal electronicdevice, a handheld device, a personal computer, a wireless local loop(WLL) station, an Internet of Things (IoT) device, an Internet ofEverything (IoE) device, a machine type communication (MTC) device, anappliance, an automobile, or the like.

In some cases, a UE 115 may also be able to communicate directly withother UEs (e.g., using a peer-to-peer (P2P) or device-to-device (D2D)protocol). One or more of a group of UEs 115 utilizing D2Dcommunications may be within the coverage area 110 of a cell. Other UEs115 in such a group may be outside the coverage area 110 of a cell, orotherwise unable to receive transmissions from a base station 105. Insome cases, groups of UEs 115 communicating via D2D communications mayutilize a one-to-many (1:M) system in which each UE 115 transmits toevery other UE 115 in the group. In some cases, a base station 105facilitates the scheduling of resources for D2D communications. In othercases, D2D communications are carried out independent of a base station105.

Some UEs 115, such as MTC or IoT devices, may be low cost or lowcomplexity devices, and may provide for automated communication betweenmachines, i.e., Machine-to-Machine (M2M) communication. M2M or MTC mayrefer to data communication technologies that allow devices tocommunicate with one another or a base station without humanintervention. For example, M2M or MTC may refer to communications fromdevices that integrate sensors or meters to measure or captureinformation and relay that information to a central server orapplication program that can make use of the information or present theinformation to humans interacting with the program or application. SomeUEs 115 may be designed to collect information or enable automatedbehavior of machines. Examples of applications for MTC devices includesmart metering, inventory monitoring, water level monitoring, equipmentmonitoring, healthcare monitoring, wildlife monitoring, weather andgeological event monitoring, fleet management and tracking, remotesecurity sensing, physical access control, and transaction-basedbusiness charging.

In some cases, an MTC device may operate using half-duplex (one-way)communications at a reduced peak rate. MTC devices may also beconfigured to enter a power saving “deep sleep” mode when not engagingin active communications. In some cases, MTC or IoT devices may bedesigned to support mission critical functions and wirelesscommunications system may be configured to provide ultra-reliablecommunications for these functions.

Base stations 105 may communicate with the core network 130 and with oneanother. For example, base stations 105 may interface with the corenetwork 130 through backhaul links 132 (e.g., S1, etc.). Base stations105 may communicate with one another over backhaul links 134 (e.g., X2,etc.) either directly or indirectly (e.g., through core network 130).Base stations 105 may perform radio configuration and scheduling forcommunication with UEs 115, or may operate under the control of a basestation controller (not shown). In some examples, base stations 105 maybe macro cells, small cells, hot spots, or the like. Base stations 105may also be referred to as gNBs 105.

A base station 105 may be connected by an Si interface to the corenetwork 130. The core network may be an evolved packet core (EPC), whichmay include at least one mobility management entity (MME), at least oneserving gateway (S-GW), and at least one Packet Data Network (PDN)gateway (P-GW). The MME may be the control node that processes thesignaling between the UE 115 and the EPC. All user Internet Protocol(IP) packets may be transferred through the S-GW, which itself may beconnected to the P-GW. The P-GW may provide IP address allocation aswell as other functions. The P-GW may be connected to the networkoperators IP services. The operators IP services may include theInternet, the Intranet, an IP Multimedia Subsystem (IMS), and aPacket-Switched (PS) Streaming Service.

The core network 130 may provide user authentication, accessauthorization, tracking, Internet Protocol (IP) connectivity, and otheraccess, routing, or mobility functions. At least some of the networkdevices, such as base station 105 may include subcomponents such as anaccess network entity, which may be an example of an access nodecontroller (ANC). Each access network entity may communicate with anumber of UEs 115 through a number of other access network transmissionentities, each of which may be an example of a smart radio head, or atransmission/reception point (TRP). In some configurations, variousfunctions of each access network entity or base station 105 may bedistributed across various network devices (e.g., radio heads and accessnetwork controllers) or consolidated into a single network device (e.g.,a base station 105).

Wireless communications system 100 may operate in an ultra-highfrequency (UHF) frequency region using frequency bands from 700 MHz to2600 MHz (2.6 GHz), although some networks (e.g., a wireless local areanetwork (WLAN)) may use frequencies as high as 5 GHz. This region mayalso be known as the decimeter band, since the wavelengths range fromapproximately one decimeter to one meter in length. UHF waves maypropagate mainly by line of sight, and may be blocked by buildings andenvironmental features. However, the waves may penetrate wallssufficiently to provide service to UEs 115 located indoors. Transmissionof UHF waves is characterized by smaller antennas and shorter range(e.g., less than 100 km) compared to transmission using the smallerfrequencies (and longer waves) of the high frequency (HF) or very highfrequency (VHF) portion of the spectrum. In some cases, wirelesscommunications system 100 may also utilize extremely high frequency(EHF) portions of the spectrum (e.g., from 30 GHz to 300 GHz). Thisregion may also be known as the millimeter band, since the wavelengthsrange from approximately one millimeter to one centimeter in length.Thus, EHF antennas may be even smaller and more closely spaced than UHFantennas. In some cases, this may facilitate use of antenna arrayswithin a UE 115 (e.g., for directional beamforming). However, EHFtransmissions may be subject to even greater atmospheric attenuation andshorter range than UHF transmissions.

Wireless communications system 100 may support mmW communicationsbetween UEs 115 and base stations 105. Devices operating in mmW or EHFbands may have multiple antennas to allow beamforming. That is, a basestation 105 may use multiple antennas or antenna arrays to conductbeamforming operations for directional communications with a UE 115.Beamforming (which may also be referred to as spatial filtering ordirectional transmission) is a signal processing technique that may beused at a transmitter (e.g., a base station 105) to shape and/or steeran overall antenna beam in the direction of a target receiver (e.g., aUE 115). This may be achieved by combining elements in an antenna arrayin such a way that transmitted signals at particular angles experienceconstructive interference while others experience destructiveinterference.

Multiple-input multiple-output (MIMO) wireless systems use atransmission scheme between a transmitter (e.g., a base station 105) anda receiver (e.g., a UE 115), where both transmitter and receiver areequipped with multiple antennas. Some portions of wirelesscommunications system 100 may use beamforming. For example, base station105 may have an antenna array with a number of rows and columns ofantenna ports that the base station 105 may use for beamforming in itscommunication with UE 115. Signals may be transmitted multiple times indifferent directions (e.g., each transmission may be beamformeddifferently). A mmW receiver (e.g., a UE 115) may try multiple beams(e.g., antenna subarrays) while receiving the synchronization signals.

In some cases, the antennas of a base station 105 or UE 115 may belocated within one or more antenna arrays, which may support beamformingor MIMO operation. One or more base station antennas or antenna arraysmay be collocated at an antenna assembly, such as an antenna tower. Insome cases, antennas or antenna arrays associated with a base station105 may be located in diverse geographic locations. A base station 105may multiple use antennas or antenna arrays to conduct beamformingoperations for directional communications with a UE 115.

In some cases, wireless communications system 100 may be a packet-basednetwork that operate according to a layered protocol stack. In the userplane, communications at the bearer or Packet Data Convergence Protocol(PDCP) layer may be IP-based. A radio link control (RLC) layer may insome cases perform packet segmentation and reassembly to communicateover logical channels. A medium access control (MAC) layer may performpriority handling and multiplexing of logical channels into transportchannels. The MAC layer may also use hybrid automatic repeat request(HARQ) to provide retransmission at the MAC layer to improve linkefficiency. In the control plane, the radio resource control (RRC)protocol layer may provide establishment, configuration, and maintenanceof an RRC connection between a UE 115 and a network device or corenetwork 130 supporting radio bearers for user plane data. At thephysical (PHY) layer, transport channels may be mapped to physicalchannels.

Wireless communications system 100 may support operation on multiplecells or carriers, a feature which may be referred to as carrieraggregation (CA) or multi-carrier operation. A carrier may also bereferred to as a component carrier (CC), a layer, a channel, etc. Theterms “carrier,” “component carrier,” “cell,” and “channel” may be usedinterchangeably herein. A UE 115 may be configured with multipledownlink CCs and one or more uplink CCs for carrier aggregation. Carrieraggregation may be used with both FDD and TDD component carriers.

In some cases, wireless communications system 100 may utilize enhancedcomponent carriers (eCCs). An eCC may be characterized by one or morefeatures including: wider bandwidth, shorter symbol duration, shorterTTIs, and modified control channel configuration. In some cases, an eCCmay be associated with a carrier aggregation configuration or a dualconnectivity configuration (e.g., when multiple serving cells have asuboptimal or non-ideal backhaul link). An eCC may also be configuredfor use in unlicensed spectrum or shared spectrum (where more than oneoperator is allowed to use the spectrum). An eCC characterized by widebandwidth may include one or more segments that may be utilized by UEs115 that are not capable of monitoring the whole bandwidth or prefer touse a limited bandwidth (e.g., to conserve power).

In some cases, an eCC may utilize a different symbol duration than otherCCs, which may include use of a reduced symbol duration as compared withsymbol durations of the other CCs. A shorter symbol duration isassociated with increased subcarrier spacing. A device, such as a UE 115or base station 105, utilizing eCCs may transmit wideband signals (e.g.,20, 40, 60, 80 MHz, etc.) at reduced symbol durations (e.g., 16.67microseconds). A TTI in eCC may consist of one or multiple symbols. Insome cases, the TTI duration (that is, the number of symbols in a TTI)may be variable.

A shared radio frequency spectrum band may be utilized in an NR sharedspectrum system. For example, an NR shared spectrum may utilize anycombination of licensed, shared, and unlicensed spectrums, among others.The flexibility of eCC symbol duration and subcarrier spacing may allowfor the use of eCC across multiple spectrums. In some examples, NRshared spectrum may increase spectrum utilization and spectralefficiency, specifically through dynamic vertical (e.g., acrossfrequency) and horizontal (e.g., across time) sharing of resources.

In some cases, wireless communications system 100 may utilize bothlicensed and unlicensed radio frequency spectrum bands. For example,wireless communications system 100 may employ LTE License AssistedAccess (LTE-LAA) or LTE Unlicensed (LTE U) radio access technology or NRtechnology in an unlicensed band such as the 5 Ghz Industrial,Scientific, and Medical (ISM) band. When operating in unlicensed radiofrequency spectrum bands, wireless devices such as base stations 105 andUEs 115 may employ listen-before-talk (LBT) procedures to ensure thechannel is clear before transmitting data. In some cases, operations inunlicensed bands may be based on a CA configuration in conjunction withCCs operating in a licensed band. Operations in unlicensed spectrum mayinclude downlink transmissions, uplink transmissions, or both. Duplexingin unlicensed spectrum may be based on frequency division duplexing(FDD), time division duplexing (TDD) or a combination of both.

A UE 115 attempting to access a wireless network may perform an initialcell search by detecting a primary synchronization signal (PSS) from abase station 105. The PSS may enable synchronization of slot timing andmay indicate a physical layer identity value. The UE 115 may thenreceive a secondary synchronization signal (SSS). The SSS may enableradio frame synchronization, and may provide a cell identity value,which may be combined with the physical layer identity value to identifythe cell. The SSS may also enable detection of a duplexing mode and acyclic prefix length. After receiving the PSS and SSS, the UE 115 mayreceive a master information block (MIB), which may be transmitted in aphysical broadcast channel (PBCH) by the base station 105. The MIB maycontain system bandwidth information, a system frame number (SFN), and aphysical HARQ indicator channel (PHICH) configuration.

After decoding the MIB, the UE 115 may receive one or more systeminformation blocks (SIBs). For example, SIB1 may contain cell accessparameters and scheduling information for other SIBs. For instance, SIB1access information, including cell identity information, and it mayindicate whether a UE 115 is allowed to camp on a coverage area 110.SIB1 also includes cell selection information (or cell selectionparameters) and scheduling information for other SIBs, such as SIB2.Decoding SIB1 may enable the UE 115 to receive SIB2, where SIB2 maycontain radio resource control (RRC) configuration information relatedto random access channel (RACH) procedures, paging, physical uplinkcontrol channel (PUCCH), physical uplink shared channel (PUSCH), powercontrol, sounding reference signal (SRS), and cell barring. DifferentSIBs may be defined according to the type of system informationconveyed. In some cases, SIB2 may be scheduled dynamically according toinformation in SIB1, and includes access information and parametersrelated to common and shared channels.

After the UE 115 decodes SIB2, it may transmit a RACH preamble to a basestation 105. For example, the RACH preamble may be randomly selectedfrom a set of 64 predetermined sequences. This may enable the basestation 105 to distinguish between multiple UEs 115 trying to access thesystem simultaneously. The base station 105 may respond with a randomaccess response that provides an uplink resource grant, a timingadvance, and a temporary cell radio network temporary identifier(C-RNTI). The UE 115 may then transmit an RRC connection request alongwith a temporary mobile subscriber identity (TMSI) (e.g., if the UE 115has previously been connected to the same wireless network) or a randomidentifier. The RRC connection request may also indicate the reason theUE 115 is connecting to the network (e.g., emergency, signaling, dataexchange, etc.). The base station 105 may respond to the connectionrequest with a contention resolution message addressed to the UE 115,which may provide a new C-RNTI. If the UE 115 receives a contentionresolution message with the correct identification, it may proceed withRRC setup. If the UE 115 does not receive a contention resolutionmessage (e.g., if there is a conflict with another UE 115), the UE 115may repeat the RACH process by transmitting a new RACH preamble.

Wireless devices in wireless communications system 100 may sendtransmissions in accordance with a certain link budget. The link budgetmay account for allowed signal attenuation between a UE 115 and a basestation 105, as well as antenna gains at the UE 115 and base station105. Accordingly, the link budget may provide, for example, a maximumtransmit power for the various wireless devices within wirelesscommunications system 100. In some cases, a UE 115 may coordinatetransmit power with a serving base station 105 to mitigate interference,improve the uplink data rate, and prolong battery life.

Uplink power control may include a combination of open-loop andclosed-loop mechanisms. In open-loop power control, the UE transmitpower may depend on estimates of the downlink path-loss and channelconfiguration. In closed-loop power control, the network may directlycontrol the UE transmit power using explicit power-control commands.Open-loop power control may be used for initial access, such as thetransmission of a physical random access channel (PRACH) by a UE 115,whereas both open and closed loop control may be used for uplink controland data transmission. A UE 115 may determine power using an algorithmthat takes into account a maximum transmission power limit, a targetbase station receive power, path loss, modulation and coding scheme(MCS), the number of resources used for transmission, and a format ofthe transmitted data (e.g., physical uplink control channel (PUCCH)format). Power adjustments may be made by a base station 105 using atransmit power command (TPC) messages, which may incrementally adjustthe transmit power of a UE 115 as appropriate.

FIG. 2 illustrates an example of a wireless communications system 200that supports mmW beam selection in accordance with various aspects ofthe present disclosure. In some examples, wireless communications system200 may implement aspects of wireless communications system 100. Forexample, wireless communications system may include a base station 105-aand a UE 115-a, which may be examples of the corresponding devicesdescribed with reference to FIG. 1. Wireless communications system 200may support beam selection based on array gains for transmit and receivebeams at UE 115-a, enabling efficient power adjustment techniques fortransmitting random access transmissions.

Wireless communications system 200 may support beamformed transmissionsbetween base station 105-a and UE 115-a. For example, wirelesscommunications system 200 may operate using multiple communication beams(e.g., in mmW frequency ranges). As a result, signal processingtechniques, such as beamforming may be used to combine energy coherentlyand, for example, overcome path losses. By way of example, base station105-a may utilize multiple antennas, and each antenna may transmit (orreceive) a phase-shifted version of a signal such that the phase-shiftedversions constructively interfere in certain regions and destructivelyinterfere in others. Weights may be applied to the various phase-shiftedversions, e.g., in order to steer the transmissions in a desireddirection. Such techniques (or similar techniques) may serve to increasethe cell coverage area 110-a of the base station 105-a or otherwisebenefit wireless communications system 200.

Base station 105-a may include downlink beams 205 for communication andUE 115-a may also include receive beams 210 for transmitting andreceiving synchronization signals. Beams 205 and beams 210 may alsorepresent examples of directional beams over which data (or controlinformation) may be transmitted and/or received. Accordingly, each beam205 may be directed from base station 105-a toward a different region ofthe coverage area 110-a and in some cases, two or more of beams 205 and210 may overlap. Beams 205 and 210 may also be utilized simultaneouslyor at different times.

In some cases, a mapping may exist between a beam 210 used to receivedownlink transmissions (e.g., a UE receive beam 210-a) and a beam 210used for sending uplink transmissions (e.g., a UE transmit beam 210-b).For example, base station 105-a may send a downlink transmission usingbeam 205-a and UE 115-a may receive the downlink transmission usingdownlink receive beam 210-b. Based on the use of receive beam 210-a forthe downlink transmission, UE 115-a may then map a corresponding uplinktransmit beam 210-b for sending an uplink transmission to base station105-a, thereby creating a beam pair. In such cases, UE 115-a may be saidto have beam correspondence. In other cases, UE 115-a may not have beamcorrespondence. For instance, base station 105-a may send a downlinktransmission using beam 205-a and UE 115-a may receive the downlinktransmission on, for example, one or more sidelobes corresponding toreceive beam 210-a, or on receive beam 210-b. UE 115-a may then useanother beam 210, that may not correspond (i.e., a mapping does notexist) to downlink receive beam 210-a when sending an uplinktransmission. In such cases, UE 115-a may have been unable to determinea beam pairing based on the angle of arrival of the downlinktransmission or based on the downlink transmission being received in adifferent direction than receive beam 210-a.

A systematic search procedure may be used to identify beams transmitbeams and receive beam pairs. Base station 105-a may transmitsynchronization signals (SS) in blocks on different downlink beams.Synchronization signals may be sent for example on transmit beam 205-a,followed by transmit beam 205-b, followed by transmit beam 205-c,followed by transmit beam 305-d and so on until all SS blocks are sent.UE 115-a may also monitor the downlink beams on different receive beams.The UE for example may attempt to receive SS transmit beams on receivebeam 210-a, followed by receive beam 210-b, followed by receive beam210-c, followed by receive beam 210-d. The UE may decode the received SSsignals and determine a Signal to Interference and Noise Ratio (SINR). Abeam list can be generated, and stored in memory, for example bydetermining which beam pairs have a SINR suitable or useful forcommunication.

In one aspect, SS blocks corresponding to different downlink beams usedby the base station 105-a may be 4 SS blocks for 0-3 GHz transmissions,8 SS blocks for 3-6 GHz transmissions and 64 SS blocks for 6 GHz andhigher transmissions.

FIG. 3 illustrates an example of a wireless communications system 300that supports mmW beam selection in accordance with aspects of thepresent disclosure. In this example UE 115-a is in cell coverage area110-a, that may be considered the UEs serving cell. UE-115 a may alsoable to receive downlink signals from base station 105-b that has adifferent cell coverage area 110-b. In this example, a systematic searchmay be used to determine beams from both gNB 105-a and gNB 105-b thatmay be placed in a beam list. In this case, UE 115-a will monitorsynchronization blocks from both gNB 105-a and gNB 105-b. In one aspect,each gNB may run through all its transmit beams and the UE may scanthrough each receive beam to identify suitable beam pairs for possibleinclusion on the beam list. In various aspects, gNB105-a and gNB105-bmay transmit their SS blocks simultaneously. In this example, gNB 105-aand gNB 105-b transmit SS blocks on each of its corresponding beamssimultaneously. UE 115-a may incrementally scan through each receivebeams searching for SS signals. The UE may then select suitable beampairs according to one or more criterion for placement on the beam list.

gNB 105-a and gNB 105-b may for example transmit on 64 SS blocksassociated with downlink beams (all downlink beams not shown) that coverin azimuth their respective cell coverage areas 110. gNB 105-a maytransmit each SS block including the four SS blocks associated with thefour consecutive transmit beams shown, transmit beam 305-a, transmitbeam 305-b, transmit beam 305-c, and transmit beam 305-d.Simultaneously, gNB 105-b may transmit each corresponding SS blockincluding the four SS blocks associated with four consecutive transmitbeams shown, transmit beams 306-a, 306-b, 306-c, 306-d. UE 115-a mayscan through its receive beams (all receive beams not shown) includingthe four consecutive receive beams shown, receive beams 210-a, 210-b,210-c and 210-d. The UE may then determine which beams should be placedin the beam list.

In one aspect, the UE may choose not to include any beams from gNB 105-ain the beam list since it is the serving cell gNB. The beams from gNB-bmay be chosen for inclusion in the beam list since they correspond todifferent clusters in the environment providing rate and diversityimprovement. In another aspect, the UE may choose to include a smallnumber of beams (e.g. 1, 2 or 3 beams) from serving cell 110-a in thebeam list. The beams may be chosen to correspond to different subarraysthereby indicating correspondence to other clusters in the channel.

In another aspect the UE can manage the number of candidate beams fromgNB-a 105 a and gNB 105-b based on their respective distances to the UE.Beam management may include limiting the range of the beams. Thesedistances may be estimated by triangulation or other methods. The UE maythen determine the number of gNB-a beams and the number of gNB-b beamsto be placed on the beam list as an inverse function of their respectivedistance from the UE.

In yet another aspect, gNB beams may be selected for inclusion on thebeam list if they allow data detection and reasonable demodulationperformance using UE PO beams. PO beams may allow the UE to consume lesspower than narrower UE beams. In another aspect the UE may request gNB105-a or gNB 105-b to use a coarser code book. This may help to reduceUE energy consumption and ACLRs levels.

It can also be appreciated that the beams selected for inclusion on thebeam list may be selected using a variety of different criteria. Othercriterion that may be used include Reference Signal Receive Power(RSRP). Reference Signal Receive Quality (RSRQ), Received SignalStrength Indication (RSSI), and/or Signal to Noise Ratio (SNR).

FIG. 4 illustrates a plot 400 showing a model of the number of beamsavailable to UEs in a serving cell by received SNR threshold in anexemplary environment. In this case, a 16×4 gNB array with 16 SStraining beams. The UEs are assumed to use PO receive beam. A 57 sectormodel using the 5G New Radio channel model was used to generate the plot400.

Referring back to FIG. 1, plot 400 shows statistically the number of SSbeams that may be available to the UEs 115-a in a single serving cellcoverage area 110-a from a single gNB 105-a. The cumulativedistributions function 410 of the number of SS beams that can bereceived above various SINR thresholds 420 is shown (i.e. −5 dB. −10 dB,−15 dB and −20 dB). It is apparent from plot 400 that many UE 115 inn asingle cell coverage area 110-a may have a large amount of beams thatmay be suitable for tracking and use in case of blockage. For example,without UE side beamforming the −15 dB PO SINR threshold (−6 dB with UEbeamforming), over seventy five percent of UEs would have more than 30useful beams. This is a large amount of beams to track, requiring alarge amount of modem and RF power. Accordingly, it can be appreciatedthat in many cases a UE may only want to select a subset of these beamsfor subsequent tracking and use.

FIG. 5 illustrates an example profile 500 of the number of useful beamson a beam list that might be available to a UE over time 511. In thisexample, a beam list may have been generated using a beam trainingprocedure. The UE may have selected four beams for inclusion on the beamlist by selecting the beams with the greatest SINR for example. The beamlist may initially have four beams that are subsequently activelytracked by the UE during a first time interval 512. During the firsttime interval 512 one of the beams may be blocked or otherwise renderednot useful. The UE may then have three suitable beams available during asecond time interval 514. The UE, still tracking the beams on the beamlist, may discern that the one beam that was blocked during the firsttime interval 512 has once again available. Thus the UE will have foursuitable beams available while tracking during a third time interval516. Once again, a single beam on the beam list may be blocked and theUE will have three beams available during a fourth time interval 518.Then a second beam may be blocked, and the UE will have only twosuitable beams available during a fifth time interval 520. Finally, athird beam may become blocked leaving only a single suitable beamavailable during a sixth time interval 522.

It can be appreciated, that as the number of suitable beams becomes lowthat it may be of benefit to do another beam training procedure andgenerate another beam list. In one aspect, a threshold is used todetermine when to do another beam training procedure. For example, whenthe number of useful beams in the beam list falls below two, the UE mayexecute another beam training procedure. In this example the UE wouldbegin beam training during the sixth tracking interval 522. Multiplebeam trainings may take place to ensure a minimum number of beams staypopulated on the list.

In various aspects, the threshold may be determined by the UE. The UEmay set the threshold based on many factors including a number of activeUEs in the cell, a UE's mobility as determined by the gNB, a geometry ofthe cell served by the gNB, or a default parameter. In other aspects thegNB may determine the threshold or communicate information fordetermining the threshold to the UE. This might include for example anumber of active UEs in the cell, a UE's mobility as determined by thegNB, a geometry of the cell served by the gNB or a default parameter.

FIG. 6 a block diagram 600 of a UE 602 according to an aspect of thepresent disclosure. A transmitter 604 is coupled to a beam tracker 606and receiver 610. The beam tracker 606 may be configured to generate abeam list 608.

The receiver 610 may receive one or more SS blocks from one or more gNB.The receiver 610 may systematically scan through multiple receive beamsand decode sequences. The receiver may also determine SINR ratios. Thebeam associated with SS block and the receive beam forming a beam pair.The beam tracker 606 determines which beam pairs should be placed on abeam list 608 for subsequent tracking refinement and reporting. The beamtracker 606 may determine which beams to place on the beam list based onvarious criteria. The criteria may be determined by the UE or byinformation received from a gNB. Transmitter 604 may be used by the beamtracker for reporting information to the gNB or for making requests suchas a request for using a coarser code book.

FIG. 7 illustrates a block diagram 700 of a gNB 702 according to anaspect of the present disclosure. A transmitter 704 is coupled to athreshold determiner 706. The threshold determiner 706 is coupled to areceiver 708.

The transmitter 704 is configured to transmit SS blocks in correspondingdownlink transmit beams. The transmitter 704 may transmit these SSperiodically generating a downlink beam sweep over its coverage areaallowing UEs to determine suitable downlink beams for receiving gNBtransmissions. The gNB 706 may also feature a threshold determiner thatprovides information to the UE about determining the minimum number ofsuitable beams the UE may track without performing a new beam searchprocedure. The receiver 708 may also receive requests from one or moreUEs, such as a request to use a coarser code book.

FIG. 8 illustrates an exemplary flow diagram of mmW beam trackingfunction in a UE. A UE may perform beam training 802 to determine whichSS blocks corresponding to downlink transmit beams the UE can receive.The UE may use a PO beam or may sequence through directional receivebeams. The downlink transmit beam and the receive beam forming a beampair. In one aspect, the UE may listen on one directional receive beamsas a gNB transmits successively on each of its downlink beams. Then theUE may switch to another directional receive beam as the gNB transmitson each of its downlink beams. The UE may then switch to the nextdirectional receive beam and repeat successively until all receive beamshave been scanned.

The UE may then generate a beam list 804. The UE may select beams forthe beam list based on a variety of criteria. SINR may be a one of thecriteria with the UE not including beams with low SINR on the beam list804. Other criterion as discussed previously may be used to determinewhether a beam is placed on the beam list (e.g. Reference Signal ReceivePower (RSRP). Reference Signal Receive Quality (RSRQ), Received SignalStrength Indication (RSSI), and/or Signal to Noise Ratio (SNR).

The UE may also determine a threshold 806 for the minimum number ofsuitable beams that it should have available. The threshold may be basedon mobility measurements, local geometry, environment information,sensor feedback, RF power consumption, and/or thermal overshoot, amongother things. The UE may also receive information about the thresholdfrom a serving cell. This information may include, a threshold, a numberof active UEs in the cell, a UE's mobility as determined by the gNB, ageometry of the cell served by the gNB, and/or a default parameter amongother things.

The UE may then track beams on the beam list 808. Tracking may includebeam refining and reporting. By tracking multiple beams the UE may beable to quickly transition to a suitable beam if the beam or beams beingused for communication are blocked or become unavailable.

The UE may also determine the number of useful beams 810 on the beamlist. As beams on the beam list become blocked or unavailable the UE maycontinue to track them in case they become available once again. As longas the threshold number of beams is still available the UE may continueto track the beams on the beam list.

The UE may also determine whether the number of useful beams is greaterthan the threshold 812. If it is not, the flow reverts back to beamtraining 802 so that the UE may generate a new beam list. The UE mayperform this step continually or periodically.

FIG. 9 illustrates an exemplary flow diagram of gNB function forgenerating a UE beam tracking threshold. The gNB may outright determinea threshold 902 that the UE should use for beam tracking purposes. Thethreshold may be the minimum number of suitable beams that a UE shouldhave available in case of blockage or in case a current link becomesunavailable. In other aspects the gNB does not calculate the thresholdbut provides information to the UE that would be helpful in determininga threshold. Some information used by the gNB (or UE) to compute thethreshold may be a number of active UEs in the cell, a UE's mobility asdetermined by the gNB, a geometry of the cell served by the gNB or adefault parameter.

Finally, the gNB may transmit the threshold or information about thethreshold to the UE 904.

FIG. 10 illustrates an exemplary flow diagram of mmW beam trackingfunction in a UE. The UE may perform beam training 1002. In one aspect,multiple gNBs transmit their SS blocks corresponding to downlink beams,simultaneously. The UE may use a PO beam or may sequence throughdirectional receive beams. The downlink transmit beam and the receivebeam forming a beam pair. The UE may decode the symbol sequence on thebeams to determine the SINR.

In one aspect, the UE may listen on one directional receive beams as agNB transmits successively on each of its downlink beams. Then the UEmay switch to another directional receive beam as the gNB transmits oneach of its downlink beams. The UE may then switch to the directionalreceive beam and repeat successively until receive beams have beenscanned.

The UE may then determine the useful beams 1004. The useful beams may befrom one or more gNBs. Useful beams may be beams with a SINR that isabove a minimum threshold.

The UE may then generate a beam list 1008 comprised of useful beams thatmay be used for communication. The beams selected for the beam list maybe based on one or more criteria. In some aspects, the UE may put beamsfrom multiple gNBs on its beam list providing rate and spatialdiversity. In some aspects, serving cell beams on the beam list may belimited to a small number (e.g. 1 to 3 beams). These beams might bechosen for example to correspond to UE beams in different subarraysindicating correspondence to other clusters in the channel. In someaspects the beam list may be generated based on UE distance to the gNBtransmitting the beam. The UE might for example favor beams that are ashort distance to the gNB with the number of beams from each gNB beingan inverse function of the distance. Various techniques such astriangulation may be used to estimate distances.

The beam list may also be populated using other criteria. For example,the UE may favor beams that allow for detection and reasonabledemodulation performance with pseudo-omnidirectional (PO) beams allowingthe UE to save power.

The UE may than track beams on the beam list 1010. Tracking may includebeam refining and reporting. By tracking multiple beams the UE may beable to quickly transition to a suitable beam if the beam or beams beingused for communication are blocked or become unavailable.

In some aspects, the UE may also request that a gNB use a coarsercodebook reducing the UE power requirements and Adjacent Channel LeakageRatio (ACLR) levels.

The UE may then determine if there are enough suitable beams availablefor each gNB 1012. If there are, the gNB may continue to track thebeams. If not, flow reverts to beam training 1002 allowing the gNB togenerate a new beam list 1008.

FIG. 11 illustrates an exemplary flow diagram of gNB function forreceiving a request to change an SS code book. A gNB may receive a codebook change request 1102 from one or more UEs. The code book changerequest might indicate that the UE would like the gNB to transmit usinga coarser code book allowing the UE to receive with a PO beam or widerbeam saving energy. The gNB in turn may change the code book 1104 inresponse to the request.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The term “exemplary” used herein means “serving as an example,instance, or illustration,” and not “preferred” or “advantageous overother examples.” The detailed description includes specific details forthe purpose of providing an understanding of the described techniques.These techniques, however, may be practiced without these specificdetails. In some instances, well-known structures and devices are shownin block diagram form in order to avoid obscuring the concepts of thedescribed examples.

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If just the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, multiple microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of at least one of A, B, or C meansA or B or C or AB or AC or BC or ABC (i.e., A and B and C). Also, asused herein, the phrase “based on” shall not be construed as a referenceto a closed set of conditions. For example, an exemplary step that isdescribed as “based on condition A” may be based on both a condition Aand a condition B without departing from the scope of the presentdisclosure. In other words, as used herein, the phrase “based on” shallbe construed in the same manner as the phrase “based at least in parton.”

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media maycomprise RAM, ROM, electrically erasable programmable read only memory(EEPROM), compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor. Also, any connection isproperly termed a computer-readable medium. For example, if the softwareis transmitted from a website, server, or other remote source using acoaxial cable, fiber optic cable, twisted pair, digital subscriber line(DSL), or wireless technologies such as infrared, radio, and microwave,then the coaxial cable, fiber optic cable, twisted pair, DSL, orwireless technologies such as infrared, radio, and microwave areincluded in the definition of medium. Disk and disc, as used herein,include CD, laser disc, optical disc, digital versatile disc (DVD),floppy disk and Blu-ray disc where disks usually reproduce datamagnetically, while discs reproduce data optically with lasers.Combinations of the above are also included within the scope ofcomputer-readable media.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notlimited to the examples and designs described herein, but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of operating a User Equipment (UE)comprising: generating a first beam list by performing a first beamtraining procedure; tracking a plurality of beams on the first beamlist; determining a number of beams on the first beam list that areuseful; comparing the number of beams on the first beam list that areuseful with a first threshold; and generating a second beam list byperforming a second beam training procedure when the number of beams inthe first beam list that are useful is below the first threshold.
 2. Themethod of claim 1, wherein the UE determines the first threshold basedon mobility measurements, local geometry, environment information,sensor feedback, RF power consumption, or thermal overshoot.
 3. Themethod of claim 1 further comprising determining the first thresholdbased on information received from a gNB.
 4. The method of claim 1,wherein the first threshold is based on a number of active UEs in thecell, a UE's mobility as determined by a gNB, a geometry of the cellserved by the gNB, or a default parameter.
 5. The method of claim 1,wherein the second beam list has at least one beam that is also in thefirst beam list.
 6. A method of operating a gNB: determining a firstbeam threshold for a UE; transmitting information about the firstthreshold.
 7. The method of claim 6 wherein the first threshold is basedon a number of active UEs in the cell, a UE's mobility as determined bythe gNB, a geometry of the cell served by the gNB or a defaultparameter.
 8. A User Equipment (UE) comprising: a receiver configured toreceive downlink signals; a processor coupled with the receiverconfigured to: generate a first beam list by performing a beam trainingprocedure; track a plurality of beams on the first beam list; determinea number of beams on the first beam list that are useful; comparing thenumber of beams on the first beam list that are useful with a firstthreshold; and generate a second beam list by performing the beamtraining procedure when the number of beams in the first beam list thatare useful is below the first threshold.
 9. The UE of claim 8, whereinthe processor determines the first threshold based on mobilitymeasurements, local geometry, environment information, sensor feedback,RF power consumption, or thermal overshoot.
 10. The UE of claim 8wherein the processor is configured to determine the first thresholdbased on information received from a gNB.
 11. The UE of claim 8, whereinthe first threshold is based on a number of active UEs in the cell, aUE's mobility as determined by the gNB, a geometry of the cell served bya gNB, or a default parameter.
 12. The UE of claim 8, wherein the secondbeam list has at least one beam that is also in the first beam list. 13.A User Equipment (UE) comprising: means for generating a first beam listby performing a beam training procedure; means for tracking a pluralityof beams on the first beam list; means for determining a number of beamson the first beam list that are useful; comparing the number of beams onthe first beam list that are useful with a first threshold; and meansfor generating a second beam list by performing the beam trainingprocedure when the number of beams in the first beam list that areuseful is below the first threshold.
 14. The UE of claim 13, wherein theUE determines the first threshold based on mobility measurements, localgeometry, environment information, sensor feedback, RF powerconsumption, or thermal overshoot.
 15. The UE of claim 13 furthercomprising means for determining the first threshold based oninformation received from a gNB.
 16. The UE of claim 13, wherein thefirst threshold is based on a number of active UEs in the cell, a UE'smobility as determined by the gNB, a geometry of the cell served by agNB, or a default parameter.
 17. The method of claim 13, wherein thesecond beam list has at least one beam that is also in the first beamlist.